Out of consideration for environmental pollution, there has been now requested clean energy in which exhaust of carbon dioxide is low as for usage of electricity. A power storage system has been put to practical use, in which electric power generated during low power consumption is charged in a secondary cell or battery and is consumed when necessary, and also the development of a fuel cell has been advanced promptly. The fuel cell is a system that allows hydrogen extracted from natural gas or the like to react electrochemically with atmospheric oxygen to generate electricity. The heat of about 100° C. wasted during electricity generation can be used for hot-water supply and/or heating. In the fuel cell, phosphate solution type is suitable for small-scale private power generation. In a secondary battery, a sodium-sulfur cell (Na—S battery) is generally applied to power storage. In the case of the sodium-sulfur cell, a carbon material for supplying electron to sulfur is used as a cathode. Since this carbon material has generally high electronic conductivity, is inert chemically and does not react with other material, it is suitable for auxiliary electrodes.
It is desirable that internal resistance of the secondary battery is low and charge and discharge efficiency thereof is high, for the purpose of power storage. In order to improve charge and discharge efficiency of the sodium-sulfur battery, β-alumina solid electrolyte itself must be brought for low resistance, and as for a cathode, it is necessary to lower contact resistance between several materials and decrease internal resistance for promoting a reaction of an active material within the cathode. As a conductor for electrodes, a material that can contain and retain molten sulphur is also desirable, whose strength is high and electric resistance is low. The felt of a carbon fiber is generally used from this respect. In JP-H08-130032, for example, a felt made of PAN carbon fiber is employed and α-alumina powder is scattered on the surface of the felt of a carbon fiber to improve the electric conductivity and decrease contact resistance between several materials. The felt made of a PAN carbon fiber has limitations on improvement of cell performance as a electric conductor when being applied alone, and also it is not enough to improve the performance in the felt even if α-alumina powder is scattered.
On the contrary, JP-H11-158737, JP-2000-306587 and JP-2001-15369 disclosed a felt of a flame-resistant fiber turned to a carbon fiber felt by high-temperature baking, reforming in the precursor or the flame-resistant fiber being carried out, instead of reforming in the carbon fiber felt. In JP-H11-158737, a flame-resistant fiber only or the mixture of a flame-resistant fiber is turned to a carbon fiber at a high carbonization yield by carbonizing it at maximum temperature 1100° C. or high. In JP-2000-306587, a felt of a flame-resistant fiber is minutely punched with needles from the one or both surface thereof to achieve degree of fiber orientation of 20% or more in the direction of a thickness, thereby the carbon fiber felt thus obtained gets less problem of blocking as compared with a standard carbon fiber felt and thus has necessary conductivity and a high heat-resistance as an anode conductor. In JP-2001-115369, a preliminary felt is formed by rough needle-punching of a flame-resistant fiber and then web is layered on one or both sides of the felt, which is punched with needles to obtain a felt of a carbon fiber, of which both bulk density and mechanical strength are high, fiber orientation in the direction of a thickness increases greatly in degree and electrical conduction property is excellent.    [Cited Reference 1] JP-A2-H08-130032    [Cited Reference 2] JP-A2-H11-158737    [Cited Reference 3] JP-A2-2000-306587    [Cited Reference 4] JP-A2-2001-115369